This invention relates to lasers, and more particularly to free electron lasers.
In a free electron laser, a relativistic electron beam is injected through a spatially periodic wiggler field. The electrons are deflected by the wiggler field, and are caused to spontaneously emit radiation. This radiation and the wiggler field then combine in an interaction region to bunch and decelerate the electrons to produce gain. An independent signal wave may also be introduced which is amplified in the interaction as described above. In addition, many free electron lasers, particularly those designed to operate at moderate voltage (0.5-2.0 MeV) and with high current density (greater than 100 A), make use of a uniform axial magnetic field in the interaction region, in addition to the above wiggler field. The axial magnetic field helps form and confine the electron beam, and can also provide gyroresonant enhancement of the effects of the wiggler magnetic field.
An important problem with these devices is that their intrinsic efficiency is often fairly low. Previous methods of improving the efficiency of the free electron laser include tapering the strength and/or period of the wiggler magnet as described in U.S. Pat. No. 4,283,687 to John M. J. Madey et al. Another prior technique of improving efficiency utilizes a depressed collector to recover energy from the spent electron beam. The above techniques of improving efficiency are mechanically and technically complex. In respect to the tapered wiggler magnet, typically a bifilar wiggler coil must be specially flared at its ends to achieve the desired tapered wiggler field.